The present invention relates to a process for the partial combustion of hydrocarbonaceous fuels to produce gases having a substantially reduced soot content.
In the chemical and energy providing industries there is a great demand for gas containing substantial amounts of hydrogen and/or carbon monoxide in various mixtures and purities. Such gas may be used, for instance, as a starting material for the manufacture of chemical products -- e.g., ammonia, alcohols, etc., -- as a reducing agent, as a clean fuel or in hydrogenation processes. A well-known and widely employed method for the preparation of such a gas is the partial combustion of hydrocarbonaceous fuels in a substantially void or hollow reactor. One of the attractive aspects of this process is its flexibility in the types of hydrocarbon fuels which can be converted to the desired gaseous products. Suitable hydrocarbonaceous fuels which may be subject to partial combustion in this non-catalytic process include normally gaseous and liquid hydrocarbons, e.g., middle distillates and light and heavy fuel oil, as well as liquid fuels mixed with solid carbon-containing particles such as the carbon soot typically obtained as a product of partial combustion. The combustion can be effected with oxygen, with air, or with air which has been enriched with oxygen. Frequently steam is added to the reaction mixture.
The gases produced by processes for partial combustion of hydrocarbonaceous fuels invariably contain a substantial amount of free carbon (i.e., soot) which is undesirable for the subsequent utilization or processing of the gases and which should therefore be removed. The removal of the soot is normally achieved by scrubbing the gases with water. The resulting dispersion of soot particles in water is then processed in order to recover substantially soot-free water and soot. The soot removal facilities are expensive and the recovered soot generally has a low value. Alternatively, the soot dispersion in water, or even the water-free soot, can be recycled to the partial combustion reactor for conversion to carbon monoxide. However, in these instances there are difficulties in the preparation of a suitable soot-containing feed for introduction into the reactor and more elaborate mixing devices and feed nozzles are required to ensure accurate control and adequate mixing of the soot-containing fuel on introduction into the reactor. Furthermore, in any event, the recycled soot is only partially converted to usable gas products due to the longer reaction times required for its conversion while at the same time causing an objectionable build-up of ash in the reactor itself and subsequent processing equipment in cases where residual fuels containing ash-forming components are employed as the fuel feed to partial combustion.
One of the primary means previously employed to optimize usable gas (hydrogen plus carbon monoxide) formation and minimize soot formation has been to optimize and accurately control the O/C ratio (atomic ratio of total oxygen to total carbon in the feed) which is introduced into the combustion reactor. With an O/C ratio of 1, hydrocarbons in the ideal case yield carbon monoxide and water. However, in actual practice, at this stoichiometric O/C ratio, significant quantities of soot are formed in addition to the desired product gases. By increasing the atomic ratio of O to C, the amount of soot formed may be reduced, but more carbon dioxide and water will be formed. If the O/C ratio is decreased, less carbon dioxide and water will be formed, but more soot will form and there will remain more uncombusted hydrocarbons. Other means such as adding steam to the combustion zone, preheating the reactants and the utilization of particular reaction zone designs and reactant mixing schemes are said to have a desirable effect on optimization of yield to usable gas and/or minimization of undesirable soot formation. However, despite all of these previous efforts, the soot formation is still significant enough to necessitate the use of elaborate gas purification schemes.
The present invention seeks to provide a more economic process wherein the soot formed on partial combustion is more effectively and efficiently converted to the desired gaseous products, the thermal efficiency of the overall process is materially increased and the water scrubbing of the partial combustion effluent with its associated problems to recover the by-product soot is substantially eliminated.